The electrophoretic effect is well known as the prior art is replete with patents and articles describing the effect. As described and discussed in the prior, the electrophoretic effect operates on the principle that certain particles, when suspended in a medium, can be electrically charged and thereby caused to migrate through the medium to an electrode of opposite charge. Electrophoretic image displays (EPIDs) implement the electrophoretic effect to produce desired images. In prior art EPIDs, colored particles, which are charged either positively or negatively, are suspended in a dielectric fluid medium that is either clear or of a color which optically contrasts with the particles. The suspension is injected into a cell comprising two parallel screen electrodes, at least one of which is transparent. The colored particles are caused to migrate to, and impinge upon, one of the screen electrodes under the application of an electric field, thereby displacing the fluid medium at that electrode creating the desired image. When the polarity of the field is reversed, the colored particles migrate to the opposite screen electrode.
For suitable example of such devices using the electrophoretic effect, reference is made to U.S. Pat. No. 4,732,830 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS and issued to Frank J. DiSanto et al. on Mar. 22, 1988. In this patent, there is disclosed an electrophoretic display apparatus which includes a planar transparent member having disposed thereon a plurality of vertically extending, electrically conductive lines defining a grid. A plurality of horizontally extending electrically conductive cathode lines are disposed on top of the vertical lines but are insulated therefrom by a thin insulating layer, thereby forming an XY matrix of electrodes. A conductive plate or anode is spaced above the line pattern and disposed therebetween in an electrophoretic dispersion of yellow pigment particles in a dark colored suspension medium. The particles are transportable within the medium.
The performance of the resulting display is strongly dependent upon the suspension stability. Colloid particles owe their suitability against instability to the fact that their surfaces are charged and, hence, repel each other. When the particles are uncharged, the dispersion is unstable. The fact that a colloidal particle bears a net surface charge is not a sufficient condition for stability because electroneutrality demands that the particle plus its immediate surrounding bear no net charge. In other words, the surface charge must be compensated by an equal but opposite counter charge, so that surface charge and countercharge together form an electrical double layer. P. Murau and B. Singer, in an article appearing in Vol. 49, No. 9 of the Journal of Applied Physics (1978) and entitled "The Understanding and Elimination of Some Suspension Instabilities in an Electrophoretic Display", indicated that when the double layer is compressed, the particles can approach each other to within a few hundred angstroms before repulsion is felt and the van der Waals attraction becomes so strong that aggregation occurs.
The interactions of particle surfaces and charge control agents in colloidal suspensions has been the subject of considerable research. Reference is made to an article entitled "Mechanism of Electric Charging of Particles in Nonaqueous Liquids" appearing in Vol. 15 of the Journal of the American Chemical Society (1982), F. M. Fowkes et al discuss the mechanism of electrostatic charging of suspended acidic particles by basic dispersants in solvents of low dielectric constant. Reference is also made to an article entitled "Steric and Electrostatic Contributions to the Colloidal Properties of Nonaqueous Dispersions" appearing in Vol. 21 of the Journal of the American Chemical Society (1984) wherein F. M. Fowkes and R. J. Pugh discuss the importance f of anchoring sites for steric stabilizers in minimizing particle flocculation. The essential point developed by these references is that particle surface interactions are acid-base in character. Acidic pigment surface sites and basic charge control agents yield negative pigment surface charge. On the other hand, basic pigment surface sites and acidic charge control agents yield positive pigment surface charge.
Since electrophoretic devices utilize low polarity liquids in which ionization of ordinary organic acids and salts is negligible the charge of the particle is governed by trace impurities unless otherwise controlled by adsorbing on the pigment surface a suitable charge control agent. This amount of charge, although sufficient for electrophoretic activity may still be inadequate for electrostatic stabilization of the suspension. If the charge control agent is also polymeric, or a polymeric dispersant is present in addition, the colloid stability can be further enhanced. See for example, an article by P. Murau and B. Singer appearing in Vol 49, No. 9 of the Journal of Applied Physics (1978) and entitled "The Understanding and Elimination of some suspension instabilities in an Electrophoretic Display". Over recent years, attention has therefore been directed to dispersion stabilization by way of adsorbed polymers on particle surfaces. If two colloidal particles coated with adsorbed layers of polymers approach each other, steric repulsion can occur as soon as the polymer layers start to penetrate. According to Murau and Singer, the polymer molecules adsorbed on a colloidal particle never lie flat on the surface. Rather, parts of the long chains (loose-ends, side branches, and loops) are free from the surface and surrounded by liquid.
As will be recognized by a person skilled in the art, the selection of the electrophoretic particles used in the EPID is very important in determining the performance of the EPID and the qualify of the viewed image produced. Ideally, electrophoretic particles should have an optimum charge/mass ratio, which is dependent upon the particle size and surface charge, in order to obtain good electrostatic deposition at high velocity as well as rapid reversal of particle motion when voltages change. Additionally, it is desirable to utilize electrophoretic particles that have essentially the same density as the fluid medium in which they are suspended. By using electrophoretic particles through the medium remains independent of both the orientation of the EPID and the forces of gravity.
To effect the greatest optical contrast between electrophoretic particles and the suspension medium, it is desirable to have either light-colored particles suspended in a dark medium or black particles suspended in a backlighted clear medium. In the prior art, it has been proven difficult to produce black electrophoretic particles that are dielectric, of uniform size and have a density matching that of a common suspension medium. As a result, EPIDs, commonly use readily manufactured light colored electrophoretic particles suspended in dark media. Such EPIDs are exemplified in U.S. Pat. Nos: 4,655,897 to DiSanto et al., 4,093,5341 to Carter et al., 4,298,4481 to Muller et al., and 4,285,801 to Chaing.
Another parameter of display performance which is of critical importance is writing speed. Writing speed is the speed at which the colored particles, resting initially on one screen electrode move through a medium and rest on an oppositely charged screen electrode when an electric field is applied. The actual writing time or speed required to produce an image on the display is very dependent upon 1) the speed at which the colored particles separate from the screen electrode and 2) the speed at which the colored particles pack at the oppositely charged screen electrode. In the quest to increase the writing speed of the display, prior art suspension formulations have been made which exhibit extremely fast speeds of separation from the screen electrode but which exhibit slow and unusable packing speeds at the oppositely charged screen electrode.
Accordingly, it is an object of the present invention to provide an electrophoretic suspension composition and method of making, which exhibits an increased writing speed via an increased speed of colored particle separation from the screen electrode while maintaining an acceptable speed of colored particle packing at the oppositely charged screen electrode.